The removal of electrons from water molecules and their transfer to NADP+ requires energy. The electrons are moving from a redox potential of about +0.82 volt in water to −0.32 volt in NADPH. Thus enough energy must be available to move them against a total potential of 1.14 volts. Where does the needed energy come from? The answer: Light.

First the energy in the light is captured by special proteins called light harvesting complexes or antennae. These special proteins contain pigments like chlorophyll which can capture the energy of photons (the particles that make up light). The light energy is converted into electrical energy (electrons moving through proteins that act like wires). This energy is funnelled into an important protein complex called the reaction center. Here some chemistry takes place and an electron is given a lot of energy and becomes excited. This special electron comes from water. When water is split into hydrogen and oxygen, an electron can be captured and excited by the electrical energy from the light harvesting complexes. The splitting of water is how photosynthesis produces the oxygen we breath

OK. So now we have an excited electron in the reaction center. This electron can be thought of as water at the top of a waterfall. As the electron becomes less excited, it releases some energy. Just as water loses energy as it falls from the top of a waterfall. The little bits of energy released are captured by another set of proteins. These convert the electrical energy of the excited electron into chemical energy. The chemical energy takes the form of high energy chemical bonds in things like ATP and NADPH.